Michele Laskowsky prepares sago pond weed seeds for planting in Mountain Lake by a team of biologists with the Presidio Trust and National Parks Conservancy in San Francisco, Calif. on Tuesday, March 25, 2014. The reintroduction of the underwater grass is part of an extensive restoration project at the lake. less

Michele Laskowsky prepares sago pond weed seeds for planting in Mountain Lake by a team of biologists with the Presidio Trust and National Parks Conservancy in San Francisco, Calif. on Tuesday, March 25, 2014. ... more

Michele Laskowsky and Jonathan Young launch a small boat before a team of biologists with the Presidio Trust and National Parks Conservancy plant sago pond weed seeds at Mountain Lake in San Francisco, Calif. on Tuesday, March 25, 2014. The reintroduction of the underwater grass is part of an extensive restoration project at the lake. less

Michele Laskowsky and Jonathan Young launch a small boat before a team of biologists with the Presidio Trust and National Parks Conservancy plant sago pond weed seeds at Mountain Lake in San Francisco, Calif. ... more

Marion Anthonisen drops a mud ball with sago pond weed seeds into Mountain Lake in San Francisco, Calif. on Tuesday, March 25, 2014. Biologists with the Presidio Trust and the National Parks Conservancy are reintroducing the underwater grass as part of an extensive restoration project at the lake. less

Marion Anthonisen drops a mud ball with sago pond weed seeds into Mountain Lake in San Francisco, Calif. on Tuesday, March 25, 2014. Biologists with the Presidio Trust and the National Parks Conservancy are ... more

Finn Black (left) and Marion Anthonisen toss a clump of mud containing sago pond weed seeds into Mountain Lake in San Francisco, Calif. on Tuesday, March 25, 2014. Biologists with the Presidio Trust and the National Parks Conservancy are reintroducing the underwater grass as part of an extensive restoration project at the lake. less

Finn Black (left) and Marion Anthonisen toss a clump of mud containing sago pond weed seeds into Mountain Lake in San Francisco, Calif. on Tuesday, March 25, 2014. Biologists with the Presidio Trust and the ... more

Mountain Lake, the Presidio's watery gem, has finally been cleansed of the sediments and toxic chemicals that poisoned it for a century, and scientists have begun restoring its natural vegetation to create a new wildlife habitat.

Working by boat along the lake's shoreline, botanists last week planted seeds and buds of long-vanished flora into the bottom - each batch encased in hand-made balls of protective mud.

"These are pioneer species," said plant ecologist Michele Laskowsky, whose team collected them from Bay Area lakes and lagoons and propagated them in the Presidio's Native Plant Nursery.

"They're wonderful habitat plants," she said, "because they're great food for ducks and other animals in the water, they help keep the water clear, they help prevent the algae blooms that reduce oxygen in the water, and they'll provide shelter for all the small native fish that will soon be living here."

From their boat, Presidio interns Finn Black and Marion Anthonisen gingerly lowered the mud balls into the lake. The mud balls had been placed inside wire cages to protect the fragile plants from the countless invasive fish that have flourished there for decades.

In stages that will take months or even years, Presidio scientists are working to restore the lake and its surrounding area to their natural condition, and those invasive fish must go.

For decades San Francisco residents have used Mountain Lake as a convenient place to dump unwanted pets from their home aquariums - often ordinary goldfish and colorful koi. All are varieties of carp, however, and all can grow to huge sizes with big appetites for smaller prey.

"There's one big orange koi that patrols the south shore," Young said. "It must be 2 1/2 feet long, but I've put a net there, and I know we'll get it."

Young sends the invasive fish he catches to a reptile rescue center in Sebastopol, where they are adopted by owners of isolated artificial vineyard ponds - "where they won't create the same issues in those nature systems as they are here," Young said.

The first plants in

The first species of native plants that Black and Anthonisen lowered to the lake bottom last week are called sago pondweed. They will be followed by coontails and water nymphs - what botanists call SAV, for "submerged aquatic vegetation." All were selected for their many benefits to the lake's restoration, Laskowsky said, and were originally collected from Marin County lakes, lagoons and oceanside coves.

The vegetation once grew naturally in Mountain Lake Park, and Laskowsky said she identified the species in the plant collections and records of the California Academy of Sciences in Golden Gate Park. They go back to the 189os, when Alice Eastwood was the museum's first paid curator of botany, Laskowsky said.

Native fish species will be introduced to the lake once it is free of all its alien fish. The first, Young said, will be the three-spined sticklebacks, a species that thrives in the Presidio's Lobos Creek. Eventually the area will see the return of still more native fish, turtles, reptiles and other wildlife, he said.

"It has to be a long process," Young said, "but then, restoring the ecology and the wildlife of a 4-acre lake to the way it was hundreds of years ago is a complex process, and it has to go step-by-step."

Cleaning complete

The state's Environmental Protection Agency gave its approval for the project this week when its Department of Toxic Substances Control noted that during 11 years of testing and two years of dredging, the Presidio had successfully removed nearly 17 tons of potentially toxic sediments from the lake.

As a result, the agency certified, Mountain Lake is now free of toxic chemicals like lead and petroleum hydrocarbons, and that its surface water meets "drinking water levels."

In a campaign to protect the lake restoration effort, the Presidio is distributing stickers to the public bearing the message "Love Mountain Lake" and a colored image of the endangered Western Pond Turtle. The sticker urges San Franciscans and park visitors to pledge not to feed the area's wildlife, not to abandon unwanted plants or animals there, and to help keep the lake healthy.